S6K directly phosphorylates IRS-1 on Ser-270 to promote insulin resistance in response to TNF-(alpha) signaling through IKK2.
S6K1 (p70S6K) is a serine kinase downstream from Akt in the insulin signaling pathway that is involved in negative feedback regulation of insulin action. S6K1 is also activated by TNF-alpha, a pro-inflammatory cytokine. However, its role remains to be characterized. In the current study, we elucidated a mechanism for S6K1 to mediate TNF-alpha-induced insulin resistance in adipocytes and hepatocytes. S6K1 was phosphorylated at Thr-389 in response to TNF-alpha. This led to phosphorylation of IRS-1 by S6K1 at multiple serine residues including Ser-270, Ser-307, Ser-636, and Ser-1101 in human IRS-1 (Ser-265, Ser-302, Ser-632, and Ser-1097, in rodent IRS-1). Direct phosphorylation of these sites by S6K1 was observed in an in vitro kinase assay using purified IRS-1 and S6K1. Phosphorylation of all these serines was increased in the adipose tissue of obese mice. RNAi knockdown demonstrated an important role for S6K1 in mediating TNF-alpha-induced IRS-1 inhibition that led to impaired insulin-stimulated glucose uptake in adipocytes. A point mutant of IRS-1 (S270A) impaired association of IRS-1 with S6K1 resulting in diminished phosphorylation of IRS-1 at three other S6K1 phosphorylation sites (Ser-307, Ser-636, and Ser-1101). Expression of a dominant negative S6K1 mutant prevented TNF-induced Ser-270 phosphorylation and IRS-1 protein degradation. Moreover, in IKK2 (but not IKK1)-null cells, TNF-alpha treatment did not result in Thr-389 phosphorylation of S6K1. We present a new mechanism for TNF-alpha to induce insulin resistance that involves activation of S6K by an IKK2-dependent pathway. S6K directly phosphorylates IRS-1 on multiple serine residues to inhibit insulin signaling.
Zhang
Jin
J
Pennington Biomedical Research Center, Louisiana State University System, Baton Rouge, Louisiana 70808, USA.
Gao
Zhanguo
Z
Yin
Jun
J
Quon
Michael J
MJ
Ye
Jianping
J
D017667:Adipocytes / Q000201:enzymology; D000273:Adipose Tissue / Q000201:enzymology; D000818:Animals; D002460:Cell Line; D055785:Gene Knockdown Techniques; D022781:Hepatocytes / Q000201:enzymology; D006801:Humans; D051550:I-kappa B Kinase / Q000235:genetics / Q000378:metabolism*; D055504:Insulin Receptor Substrate Proteins / Q000235:genetics / Q000378:metabolism*; D007333:Insulin Resistance / Q000235:genetics; D051379:Mice; D008820:Mice, Obese; D010766:Phosphorylation / Q000235:genetics; D017354:Point Mutation; D038744:Ribosomal Protein S6 Kinases, 90-kDa / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction / Q000235:genetics; D014409:Tumor Necrosis Factor-alpha / Q000235:genetics / Q000378:metabolism*
C526292:IRS1 protein, human; D055504:Insulin Receptor Substrate Proteins; C526295:Irs1 protein, mouse; D014409:Tumor Necrosis Factor-alpha; C069329:RPS6KA1 protein, human; D038744:Ribosomal Protein S6 Kinases, 90-kDa; C512964:Rps6ka1 protein, mouse; D051550:I-kappa B Kinase